1. Field of the Invention
The present invention relates to an image forming apparatus that detects the density or color value of an image formed on a printing material and forms an image corresponding to the detection result, and a control method thereof.
2. Description of the Related Art
In recent years, a color image forming apparatus, such as a color printer or a color copying machine, is required to improve the quality of an output image. In particular, the tone of the density of an output image and its stability are important factors to decide the image quality.
However, the color image forming apparatus changes the density or tint of an output image due to environmental variations or variation factors of various units of the apparatus caused by long-term use. In particular, an electrophotographic color image forming apparatus may change the density or tint of an output image due to slight environmental variations and disturb the image forming characteristics, and therefore needs to include a method for always maintaining a predetermined density. One such method uses a sensor (to be referred to as a “color sensor” hereinafter) for detecting a color value. More specifically, the color value of each color toner image (to be referred to as a “patch” hereinafter) formed and fixed on a printing material for color value detection is detected by a color sensor, and the amount of a toner to be transferred to the printing material is adjusted/corrected based on the detection result.
When detecting the color value of a patch using the color sensor, it is necessary to detect the color value (white reference) of white serving as a reference using a white reference plate, and to calibrate the color sensor output based on the detection result. This is because, for example, the time degradation of the light-emitting unit or the light-receiving unit of the color sensor, a change in the ambient temperature, and dust or toners adhering to the sensor surface at the time of passage of the printing material near the sensor causes the sensor output to vary even for the same patch. When detecting the color value of a patch, the white reference is acquired using the white reference plate, and the patch detection result is corrected using the acquired white reference, thereby acquiring the color value of the patch independently of the change in the sensor output value.
However, like the sensor, the white reference plate cannot be used as a reference plate for sensor output calibration if dust or toners are adhering to it. To solve this problem, Japanese Patent Laid-Open No. 2008-278215 describes a method of detecting a blot on the white reference plate by causing the color sensor to detect the color value of specific reference paper that has a known color value. In this method, the color value of the reference paper detected by the sensor calibrated using the white reference plate is compared with the known color value of the reference paper, thereby detecting a blot on the white reference plate.
However, the conventional method needs to always use the reference paper which has a known color value. If paper whose color value is different from the known color value of the reference paper—such as paper other than the reference paper or blotted reference paper—is used, the blot on the white reference plate cannot properly be detected, and color correction cannot properly be performed.